All-pass network amplifier



ALL-PASS NETWORK AMPLIFIER Original Filed Oct. 26, 1954 FIG. 2

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x B C R3 1 I I I i INVENTOR D Q- 2 e0 MAURICE A. MEYER R e e W miwzw 2 0 7| in W- I l l By NW ATTORNEY ALL-PASS NETWORK AMPLIFIER Maurice A. Meyer, Natick, Mass, assiguor to Laboratory For Electronics, Inc.,Boston, Mass, a corporation of- Delaware Original application Oct. 26, 1954, Ser.. No. 464,697; Divided and this application Mar. 12, 1956, SerrNo. 571,037' V 11 Claims. (Cl. 330-69) This invention relates in general to phase-shift circuits and in particular to a novel all-pass circuit which imparts a selected phase-shift to an input signal functionally related to its frequency, the amplitude of the output signal being. substantially independent of frequency.

This application is a division of the co-pending appli cation of Maurice A. Meyer entitled, Frequency and Phase Sensitive Apparatus, Serial No. 464,697, filed October 26, 1954. v a

All-pass networks, having amplitude and phase-shift characteristics which are respectively, independent of and dependent upon frequency, are well known in thearti however, the design of predetermined phase-shift'characteristics frequently involve numerous complex computations which yield a network configuration of nonstandard circuit parameters requiring careful adjustment to provide the desired all-pass characteristic.

Accordingly, it is a primary object of the present invention to provide a relatively simple all-pass circuit which may be designed to have a predeterminedphase shift characteristic as a function of frequency with a minimum number of computations and which may be readily constructed without requiring critical adjustment of circuit parameters.

Another object of the invention is to provide an allpass network having a selected phase-shift characteristic and providing an output signal having an amplitude independent of frequency, extending down to zero frequency.

A further object of the invention is to provide an allpass network having a phase-shift characteristic which is'controlled by a simple RC circuit. V

Basically, the invention comprises a differential amplifier having first and second inputs appropriately energized. A first impedance couples an input terminal to a first input of the differential amplifier, the input terminal being resistively coupled to the second input thereof. The output of the differential amplifier is coupled to the second input by a feedback impedance. By appropriate choice of circuit parameters, the foregoing circuit ar rangement provides an all-pass characteristic having the desired phase-shift relation.

These and other objects and advantages will become apparent from the following specification when read in connection with the accompanying drawing in which:

Fig. 1A is a combined block-schematic circuit diagram of one form of the all-pass circuit;

Figs. 1B and 1C illustrate the amplitude characteristics of the circuit of Fig. 1A;

Fig. 1D is a variation of the circuit shown in Fig. 1A; and

Fig. 2 is a complete schematic circuit diagramof the preferred embodiment of the invention.

The accompanying drawing is a reproduction of Figs. 7 and 8 of the aforementioned co-pending application.

and phase rates Patent Accordingly, the the reference characters there used are carried over into the present application.

Referring now to Fig. 1A, a schematic diagram shows the basic principles of the novel all-pass phase-shift network. In the following discussion it will be shown that the complex voltage ratio e1 v v has an amplitude characteristic independent of frequency, shown in Fig. 1B, and the phase-shift characteristic as a function of frequency of Fig. 1C.

To simplify the analysis of the circuit, capacitor C will be considered zero at first and the principle of superposition applied, first finding the output e at terminal I for e =0, e =e,; and then, e =e e =0.

From an examination of Fig. 1A, e the second input to the differential amplifier may be expressed:

Now if R -R and GR R then the output where 2 is the output for the first case.

For the second case e =0, e =e and R is in parallel same approximations as before when R -R 'and GR R,,

R1132 Man R1 R2 R1 R1+ R2 Patented July 19,1960.

the combined output R r+ 2= o This requires:

Now if terminal H is driven from a low-impedance source,

Ra e =e and 6 1 m; a 'jwC nc jtc This permits an all-pass characteristic to be obtained with the circuit herein disclosed provided Z and Z are properly chosen. For a description of a procedure for properly choosing these irnpedances, reference is made to a paper entitled, Design of RC Wide-Band 90-Degree Phase-Difference Network by D. K. Weaver, appearing in the proceedings of the I.R.E., April 1954. For example, Fig. 1D shows a modification of the circuit illustrated in Fig. 1A. In the modified circuit, the capacitor C and resistor R exchange positions. An all-pass characteristic as shown in Fig. 1B and Fig. 1C. is. provided except that the phase of the output signal leads that of the input signal whereas a lagging characteristic is represented in Fig. 1C.

The preceding discussion of Fig. 1 should facilitate the understanding of the corresponding schematic circuit diagram shown in Fig. 2 to which the reference symbols of Fig. 1 are carried over.

Referring now to Fig. 2, it is apparent from Fig. that resistor R and capacitor C control the shape of the phase-shift characteristic.

Difierential amplifier 71 comprises V7, V8, V9, V110, and V11. V10 and V9 act as plate load resistors for V7 and V8 to help maintain stable D.-C. amplification. V11 is a cathode follower which permits the output at terminal I to be taken at low impedance. Capacitor '72 provides an alternating current negative feedback path for gain stabilization and reduction of distortion. Potentiometer 73 balances the amplifier and is adjusted so that e =O when (2 :0.

Thus the phase-shift amplifier herein disclosed provides the characteristics of an all-pass network with a highly stabilized gain down to DC. Numerous modifications of and departures from the specific embodiment described herein may be practiced by those skilled in the art without departing from the disclosed inventive concepts. Consequently, the invention is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

1. An all-pass circuit comprising, differential combining means having first and second inputs and an output, frequency-sensitive attenuating means for coupling an input signal to said first input, means for attenuating said input signal by a factor of substantially two and applying the attenuated signal to said second input, and means for feeding back a portion of the output signal from said output to said second input.

2. An all-pass phase-shift network comprising, an input terminal, a differential amplifier having first and second inputs, a common terminal, a first frequency-sensitive impedance connected between said input terminal and the first input of said differential amplifier, a second impedance connected between said first input and said common terminal, a first resistance connected between said input terminal and the second input to said differential amplifier, a second resistance connected between said second input and said common terminal, and a third resistance coupling the output of said differential amplifier to the second input thereof, said first and second resistances being approximately equal and the product of the gain of said differential amplifier and said third resistance being markedly greater than the value of said second resistance.

3. Apparatus as in claim 2, wherein said first impedance is a capacitor, and said second impedance is a resistor.

4. Apparatus as in claim 2, wherein said first impedance is a resistance and said second impedance is a capacitance.

5. Apparatus for providing an output signal at an output terminal in response to an input signal, the relative phase between and amplitude ratio of said input and output signals being respectively dependent upon and independent of the input signal frequency, comprising, first means for attenuating said input signal by a factor dependent upon its frequency to provide a first attenuated signal, second means for attenuating said input signal by a factor of substantially two independent of its frequency to provide a second attenuated signal, and means for differentially combining said first attentuated signal with the cumulative combination of said second attenuated signal and a portion of the signal appearing at said output terminal, the output of said differential combining means being coupled to said output terminal.

6. Apparatus for providing an output signal in response to an input signal, the relative phase between and ratio of said signals being respectively dependent upon and independent of the input signal frequency, comprising, a differential amplifier having first and second inputs and an output, a resistive attenuator having a gain of substantially one-half for coupling said input signal to said second input, a frequency sensitive attenuating network for coupling said input signal to said first input, and a feedback impedance between said output and second input.

7. Apparatus for providing an output signal in response to an input signal, the relative phase between an amplitude ratio of said signals being respectively dependent upon and independent of the input signal frequency, comprising, an input terminal, a differential amplifier having first and second inputs and an output, a common terminal, a frequency-sensitive attenuating network connected between said input terminal and said common terminaland coupling said input terminal to said first differential amplifier input, a frequency-insensitive attenuating network having a gain of substantially one-half connected between said input terminal and said common terminal and coupling said input terminal to said second differential amplifier input, and a substantially frequencyinsensitive feedback impedance coupling said differential amplifier output to said second differential amplifier input.

8. Apparatus in accordance with claim 7 wherein said frequency-sensitive attenuating network is a resistance serially-connected to a capacitance.

9. Apparatus in accordance with claim 8 wherein said differential amplifier comprises, first, second, third and fourth electron tubes each having at least a plate, grid and cathode, a resistance common to the cathode circuit of said first and second electron tubes, said grids of said first and second electron tubes comprising said first and second differential amplifier inputs respectively, said cathodes of said third and fourth electron tubes being respectively connected to the plates of said first and second electron tubes, respective load resistances coupled to said plates of said third and fourth electron tubes, and a cathode follower for coupling said plate of said fourth electron tube to said differential amplifier output.

10. An all-pass phase-shift circuit comprising, an input terminal, a high gain differential amplifier having first and second inputs and an output, a common terminal, a frequency-sensitive attenuating network connected between said input terminal and said common terminal and coupling said input terminal to said first differential amplifier input, a first resistance connected between said input terminal and said second differential amplifier input, a second substantially equal resistance connected between said second diiferential amplifier input and said common terminal, and a third resistance connected between said output and said second difierential amplifier input, the ratio of said third resistance tosaid first resistance being substantially equal to the ratio of the diiference between said first and second resistances to said means for difierentially combining said first signal and a signal formed by combining said second signal and a signal derived from said output terminal, the output of said difi'erential combining means being coupled to said output terminal.

References Cited in the file of this patent UNITED STATES PATENTS 2,587,919 Straus et al. Mar. 4, 1952 2,677,729 Mayne May 4, 1954 2,747,028 Clark May 22, 1956 2,748,201 McMillan May 29, 1956 2,775,657 Van Zelst Dec. 25, 1956 2,780,682 Klein Feb. 5, 1957 2,796,468 McDonald June 18, 1957 2,846,522 Brown Aug. 5, 1958 2,863,122 Finkel et a1. Dec. 2, 1958 OTHER REFERENCES Lukaszewicz: A Simplified Solution [of Algebraic Polynomials, Bulletin de LAcademie Polonaise des Sciences (1953) C1. IV, vol. 2, No. 3, pages 103 to 107. 

